This method will eventually return the correct subclass of SCMWrapper. Initially, there will only be SVNWrapper.

gclient_scm: add test for unsupported scm

gclient_scm: make SCMWrapper constructor parameters non-optional

gclient_scm: add a checks for unsupported scm and unimplemented method

git_scm: pull out svn code into SVNWrapper

gclient_scm: create a create_scm factory method

TEST=
BUG=none

Review URL: http://codereview.chromium.org/216014

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@26602 0039d316-1c4b-4281-b951-d872f2087c98